Surge anoxic mix sequencing batch reactor systems

ABSTRACT

Wastewater treatment systems which utilize an interacting surge anoxic mix zone for facilitating nitrogen removal and an aerobic sequential batch reaction, clarification and decantation zone for facilitating aeration for BOD removal and nitrate production for the surge anoxic mix zone. Sludge reduction may also be accomplished by anaerobic recycle.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 09/297,729filed Dec. 2, 1999, now U.S. Pat. No. 6,398,957 issued Jun. 4, 2002,which application is a continuation-in-part of application Ser. No.09/034,512 filed Mar. 4, 1998, now U.S. Pat. No. 6,190,554 issued Feb.20, 2001, and claims priority of provisional application No. 60/102,864filed Oct. 2, 1998 and is a 371 of PCT Application US99/04744 filed Mar.3, 1999 which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention is directed to methods and apparatus forwastewater treatment, and more particularly, is directed to sequentialbatch reaction methods and apparatus for wastewater treatment.

BACKGROUND OF THE INVENTION

Wastewater treatment and treated effluent goals and standards havebecome increasingly stringent for the economical removal of wastewatercomponents such as total suspended solids (TSS), biological oxygendemand (BOD), nitrogen (as nitrate and ammonia) and phosphorous fromlarge volumes of municipal and industrial wastewater. Activated sludgesystems of either the continuous flow type in which an influent streamis continuously treated and continuously discharged through one or moretreatment zones, or the sequencing batch reactor type in which acontinuous influent stream is sequentially treated and intermittentlydischarged, are conventionally used for wastewater treatment. In suchactivated sludge treatment systems, treatment microorganisms areconcentrated in the treatment system in order to more rapidly remove thewastewater impurities, including BOD, nitrogenous, and phosphorouscomponents of the wastewater. The highly diverse, mixed culturesutilized in such activated sludge wastewater treatment systems forbiological removal of BOD, nitrogen and phosphorous include ordinaryheterotrophs (which can consume organic wastewater components to producecarbon dioxide and reduce BOD, as well as mediate denitrification),autotrophs (which mediate nitrification in consuming nitrogenouswastewater components) and phosphotrophs (which can accumulatepolyphosphates in consuming phosphorous-containing wastewatercomponents).

The various types of microorganisms in activated sludge culturestypically utilize different nutrient, oxygenation and other conditionsfor optimum removal of different wastewater components. The organicmaterials in the wastewater are consumed by “activated sludge”microorganisms for both energy and cell synthesis, driven by biologicaloxidation-reduction reactions involving transfer of electrons from awastewater component to be oxidized (the electron donor) to an oxidizingmaterial (the electron acceptor). Heterotrophic metabolism utilizesorganic wastewater components as electron donors, while autotrophicmetabolism utilizes inorganic wastewater components as electron donors.In aerobic systems in which the wastewater is aerated, oxygen isutilized by “activated sludge” microorganisms as the terminal electronacceptor. In anoxic systems, the oxygen is substantially depleted, and“activated sludge” microorganisms utilize nitrates and nitrites as theprimary terminal electron acceptors. Under anaerobic conditions, oxygen,nitrate and nitrite components are substantially depleted, andcarbonates and sulfates serve as primary terminal electron acceptors inthe cell reactions (M. G. Mandt and B. A. Bell “Oxidation Ditches”, 169pgs., 1982, Ann Arbor Science Publishers). It should be noted thatdifferent microorganisms and/or metabolic pathways may predominate undersuch different aerobic, anoxic and anaerobic conditions.

Sequencing batch reactors such as described in U.S. Pat. No. 4,596,658to Mandt, are conventionally utilized for wastewater treatment toprovide high quality effluent by subjecting a given volume of wastewaterto a predetermined sequence of different treatment steps in batch mode,in the same batch reactor equipment. In this regard, a volume ofwastewater may typically be introduced as a continuous or discontinuousfeed stream into a sequencing batch reactor treatment system andsubjected to extensive mixing and aeration for a predetermined period oftime to provide biological oxidation, consumption or other removal ofwastewater components. The mixing and aeration may subsequently bestopped and the wastewater maintained in a quiescent state in the sametreatment zone to permit wastewater solids, including microbiologicaltreatment organisms, to settle in the reactor. A clarified portion ofthe treated wastewater may be subsequently removed from the upperportion of the reactor, which in turn may be conducted to subsequenttreatment and discharge steps. Additional wastewater which is to betreated may then be introduced into the sequencing batch reactor, andthe cycle repeated. For many wastewater treatment applications,sequencing batch reactors may provide a number of advantages over oldertype continuous flow treatment systems in terms of expense, physicalarea and operating energy requirements. However, although sequencingbatch reactors have proven to be efficient, flexible and economicwastewater treatment systems, further improvements which could increasethe processing efficiency, and/or optimize treatment conditions, such asanoxic and aerobic treatment conditions, for wastewater componentremoval would be desirable. Such improved sequencing batch reactormethods and apparatus would be desirable which would be simple andeffective in operation, which would permit enhancement and synergisticinteraction of anoxic and aerobic treatment conditions for assistingwastewater component removal, and which would enhance the utility andcost effectiveness of sequencing batch reactors for wastewatertreatment.

Accordingly, it is an object of the present invention to provide suchimproved methods and apparatus and sequencing batch reactor systemswhich utilize such methods and apparatus.

In many biological treatment plants treating municipal wastewater,approximately 1 to 2% of the influent by volume exits the treatmentprocess as dilute waste sludge (WAS) requiring further treatment and/ordisposal. The further treatment and disposal of this 1 to 2% dilutewaste sludge may represent a significant part (e.g., up to 50%) of thetotal cost of wastewater treatment in a modern treatment plant. Inaddition to the capital costs for tankage and equipment for sludgereduction, dewatering, hauling, and ultimate disposal, there aresignificant continuing operating costs for power, treatment chemicals,hauling and landfill fees. The continuing operating costs for sludgereduction, dewatering, hauling and ultimate sludge disposal may evenconstitute the most substantial portion of the cost in municipalwastewater operating budgets. Furthermore, these costs have tended toincrease in recent years with increasing public and political oppositionto hauling and disposal of sludge in many localities, thereby limitingdisposal sites and capacities.

Many conventional municipal wastewater treatment plants process wastesludge by using anaerobic or aerobic digestion for pathogen and organicsludge reduction in the waste sludge produced by suspended growthbiological wastewater treatment systems, such as the various continuousflow activated sludge systems, sequencing batch reactor systems, andfixed growth biological systems including trickling filters or rotatingbiological contactors. Regardless of the source, the waste sludge (WAS)is typically dilute, generally less than 1-2% solids content by weight.The total suspended solids (TSS) contained in such sludge consists oforganic or volatile suspended solids (VSS) and inorganic, inert or fixedsuspended solids (FSS). The organic fraction is typically about 70% ofthe total suspended solids and comprises microorganisms, cellulose, bitsand pieces of plastic, and other insoluble organic compounds. Dependingon influent constituents and the type of biotreatment system used totreat the sewage, VSS will typically range from about 60% to 90% of TSS.Most larger wastewater treatment plants, and substantially all small andmedium size wastewater treatment plants, use aerobic sludge digestionrather than the more complex anaerobic digestion. In aerobic digestion,the waste sludge is held in a tank or tanks where it is repetitivelyaerated and thickened by gravity settling and decanting of supernatant.The supernatant may be recycled to the sewage processing biotreatmentplant. The remaining digested sludge is highly hydroscopic, and as apractical limit generally cannot readily be thickened beyond 2-3% byweight solids concentration.

The United States Environmental Protection Agency (the EPA) recommendsthat the waste sludge be held and aerated long enough to destroy 38% ofthe VSS conetnt in order to reduce pathogens and odor potential of thesludge, and to produce a more stable sludge which is suitable for liquidhauling and land disposal or further dewatering and processing.Dewatering may be accomplished by chemical treatment using relativelylarge doses of expensive, synthetic polymers to counteract thehydroscopic nature of the sludge, agglomerate the solids and allowfurther water separation. Horizontal, solid-bowl centrifuges or beltfilter presses are typically used to mechanically separate water fromthe polymer-treated sludge, increasing solids content of the sludge totypically 15 to 25% by weight. At this point, the sludge is truckableand can be hauled to a landfill. Alternatively, sludge drying andincineration or composting have been used to further process the sludgeto reduce its volume.

Achieving the U.S. EPA-recommended 38% reduction of VSS by aerobicdigestion typically requires considerable tankage, as well as extendedaeration contact or retention time. Tankage requirements may be, forexample, about 25% to 50% of the tankage volume for the main sewagetreatment system. In this regard, a plant treating 1 million gallons perday (MGD) of municipal sewage containing 200 mg/l of BOD5 and 200 mg/gTSS in the influent may produce about 1700 pounds per day of wastesludge. If the sludge is removed or “wasted” at 1% solids content,roughly 20,000 gallons per day (gpd) of waste sludge must be wasted fromthe treatment plant, which amounts to approximately 2% of the influentflow. Assuming 30 days sludge holding time is required for the aerobicdigestion of the sludge to insure removal of at least 38% of the sludgeVSS, the required aerobic digestion tankage of 600,000 gallons mayapproach or equal the tankage requirements for the actual sewagetreatment. Some states such as Iowa, which prohibit land application inwinter when the ground is frozen, require 180 days of sludge storage,which significantly increases the tankage requirements. In this example,of the 1700 pounds per day of sludge requiring aerobic digestion,roughly 70%, or 1190 pounds, may be organic (VSS), leaving 510 pounds ofinorganics or biologically inert materials which cannot be biologicallyoxidized. If 38% of the VSS is consumed or destroyed, there will stillbe roughly 738 pounds of VSS in the sludge. The digested sludge at thatpoint will be roughly 60% organic and 40% inorganic. Sludge is digestedand consumed (destroyed) by biological oxidation of organics and autooxidation of microbial biomass. If digested sludge leaves the digesterat 1.5% solids content, roughly 10,000 gallons per day, or 1% of theinfluent wastewater to the treatment plant, must be wet hauled to landdisposal or sent to further processing. Accordingly, it is an objectiveof some embodiments of the present disclosure to provide treatmentsystems which can substantially reduce the amount of sludge which mustbe disposed of by landfill or further processing.

Other objectives of various optional embodiments of the presentdisclosure are to provide treatment systems and processes which containsurface scum and quiescently transfer wastewater, and/or which arecapable of reducing the total amount of sludge for disposal from about1% or more to less than 0.011% by volume of influent wastewater to betreated. A further objective of such embodiments is to produce a stable,relatively inert byproduct having improved, “less sludge-like”characteristics for ultimate disposal on site or in local landfills.

These and other objects of the disclosure (which may each be independentof other objectives in different embodiments of the invention, or may becombined with other objectives, particularly in preferred embodiments),will become more apparent from the following detailed description andthe accompanying drawings.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic process flow diagram of an embodiment of a surgeanoxic mix, sequencing batch reaction process in accordance with thepresent invention in which wastewater is treated in interacting anoxicand aerated batch treatment zones;

FIG. 2 is a schematic process flow diagram of another embodiment of thepresent invention similar to that illustrated in FIG. 1, in which mixedliquor from one surge anoxic mix zone alternately interacts with twoaerated batch reaction and decantation zones,

FIG. 3 is a plan view of a dual treatment reservoir sequencing batchreactor wastewater treatment plant, utilizing a surge anoxic mixtreatment design in accordance with an embodiment of the presentinvention;

FIG. 4 is a cross sectional side view, partially broken away, of thesurge anoxic mix, sequencing batch wastewater treatment plant of FIG. 3taken through line A—A;

FIG. 5 is a cross sectional side view of the surge anoxic mix chamberand the sequencing batch reaction chamber of the treatment plant of FIG.3 taken through line B—B of FIG. 4,

FIG. 6 is a plan view of another embodiment of a sequencing batchreactor system of compact design in which a single surge anoxic mixchamber interacts with a single sequencing batch aeration and decanterchamber;

FIG. 7 is a cross sectional view of the sequencing batch reactor systemof FIG. 6 taken through line A—A;

FIG. 8 is a cross-sectional side view of an embodiment of an improvedsurface skimming and intertank flow baffling and diffusion device forwastewater treatment systems, such as the surge anoxic mix systemsillustrated in FIGS. 1-7;

FIG. 9 is a perspective view of the flow baffling and quiescentdiffusion apparatus of FIG. 8;

FIG. 10A is a plan view of the flow baffling and quiescent diffusionapparatus of FIG. 9 during high flow conditions during a decant step inthe operation of a surge anoxic mix wastewater treatment system;

FIG. 10B is a plan view of the device of FIG. 9 during the interact stepof a surge anoxic mix treatment system;

FIG. 11A is a schematic flow diagram depicting a typical solids balancefor a conventional wastewater treatment system;

FIG. 11B is a schematic flow diagram depicting a solids balance forcertain embodiments of systems in accordance with the presentdisclosure;

FIGS. 12A, and 12B represent, respectively, schematic flow diagrams ofintegrated surge anoxic mix systems with recyclic anaerobic sludgereduction, and FIGS. 12C and 12D represent independent sludge reductionsystems, utilizing anaerobic recyclic pretreatment, and including suchsludge reduction systems with inorganic content removal;

FIG. 13 is a cross sectional side view of a sludge reduction systemuseful in flow processing designs such as those of FIG. 12; and

FIG. 14 is a top view of a sludge reduction system useful in flowprocessing designs such as those of FIG. 12.

SUMMARY OF THE INVENTION

Generally in accordance with the present invention, sequencing batchreaction wastewater treatment methods and apparatus are provided whichutilize interacting anoxic mix and sequencing batch aerobic reactionzones for treating wastewater to reduce its solids content, biologicaloxygen demand and nitrogenous content, through the use of separate,sequentially interacting anoxic and aerobic treatment zones. Inaccordance with such methods, influent wastewater to be treated may beintroduced into an anoxic waste liquid treatment zone containing wastesolids including treatment microorganisms. Typically, the overall cycletime for carrying out the sequential processing step of the treatmentcycle will be less than 20 hours, and preferably less than 15 hours,although the cycle time will vary depending on factors includingtemperature and type and concentration of impurities to be treated. Theinfluent wastewater may typically be municipal or industrial wastewaterwhich may contain various types of impurities such as ammonia, organicnitrogen, nitrates, nitrites, soluble and insoluble hydrocarbons,cellulose fibers, settleable and colloidal solids and other organicmaterials, inorganic solids or grit, fats, oils, grease and phosphates,as well as a variety of other impurities. The wastewater may beintroduced directly into the anoxic treatment zone, but also may bepretreated by filtering, screening, degritting, primary clarification,and/or passage through an anaerobic treatment or retention zone, such asa “trash trap” solids collection chamber before introduction into theanoxic treatment zone. Benefits of recyclic anaerobic pretreatment willbe further described after describing surge anoxic mix treatment.

In accordance with various aspects of the present invention, wastewaterand treatment microorganisms from the anoxic waste liquid treatment zoneare introduced into an aerobic sequencing batch reactor (SBR) aerationtreatment zone containing waste solids including treatmentmicroorganisms which is preferably maintained for at least the majorportion of the treatment cycle under aerobic conditions. The wasteliquid in the aerobic sequencing batch aeration treatment zone is mixedand aerated during an aeration treatment cycle time to reduce thebiological oxygen demand of the wastewater and to convert at least aportion of the nitrogenous wastewater components to inorganic nitrate ornitrite components. As indicated, wastewater is preferably maintainedunder aerobic conditions in the aeration treatment zone for at leasthalf of the overall treatment cycle time, although less aeration timemay be required for certain types of influent wastewater, such aswastewater with relatively high nitrate content. It is an importantaspect of the methods that waste liquid and treatment microorganismsfrom the sequencing batch aeration zone are also introduced into andmixed in the anoxic mix treatment zone, to provide nitrate or nitriteoxidizing components for the anoxic treatment microorganism metabolismand to convert the nitrate or nitrite components to nitrogen for removalfrom the wastewater in the anoxic treatment zone. Subsequently, thewaste liquid in the aerated treatment zone is maintained in a quiescentstate for a settling period to form a clarified upper layer, and astratified lower layer containing waste solids including treatmentmicroorganisms. A portion of the clarified upper layer of treatedeffluent may be removed, preferably by decantation from the sequencingaeration treatment zone. The decanted clarified liquid is a high qualitytreated effluent. The influent wastewater to be treated is generallyproduced on a continuous basis, the flow rate of which may vary atdifferent times of the day with additional weekly, seasonal and othervariations. The influent wastewater is accordingly introduced into theanoxic waste liquid treatment zone, which serves to at least partiallybuffer the influent flow during the treatment cycle of the sequencingbatch reactor system. Influent flows are frequently continuous, althoughthey may vary in flow rate. Influent flow may also be discontinuous(interrupted) but will usually be continual, requiring ready treatmentcapacity. It is an advantage of surge anoxic mix systems that continuousand continual influent flows may be readily accommodated. The ratio ofthe volume of the anoxic waste liquid treatment zone to the volume ofthe aerobic sequential batch treatment zone is related to the flowbuffering or equalization function of the anoxic mix zone, and therelative ratio of nitrogen to BOD components in the wastewater.Typically, the volume ratio of the anoxic mix treatment zone to theaerobic batch treatment zone or zones which it feeds and interacts withwill be in the range of from about 0.2 to about 1.0, and preferably fromabout 0.3 to about 0.7. For municipal wastewater, the surge anoxic mixtreatment zone will typically be about one half of the volume of theaerated sequencing batch treatment zone, although for high nitrogen(e.g., concentration of 40 mg/l) industrial or municipal wastewater, thevolume of the anoxic treatment zone may be about the same as that of theaerobic sequencing batch treatment zones in order to appropriatelyprocess the nitrogen removal. As indicated, the interaction of wasteliquid between the aeration treatment zone and the anoxic mix zone isimportant. The interaction should be sufficiently rapid to be effective,but should not be so rapid that the respective anoxic and aerobicoptimized reaction gradients in the respective zones are not maintained.The introduction of wastewater and treatment microorganisms from theanoxic waste liquid treatment zone to the aerobic sequencing batchtreatment zone and the introduction of wastewater and treatmentmicroorganisms from the sequencing batch aeration zone to the anoxic mixtreatment zone are each desirably carried out during this interaction,at a rate of at least about 20 percent, and preferably at least about 50percent of the total volume of the aerobic treatment zone, per hour. Theinteraction of the waste liquid in the aerobic treatment zone iscontinued until a desired level of BOD and nitrogen reduction isachieved. Following the interaction phase, the wastewater in the aerobictreatment zone is maintained in a quiescent condition to clarify anupper layer of the wastewater, and a portion of the clarified upperlayer is discharged from the treatment zone as treated waste water. Thesequential batch treatment cycle is subsequently repeated.

Particularly preferred embodiments of the present method utilize arepetitive sequence of the following steps:

a fill step in which the influent waste water is introduced into theanoxic mix zone and pumped from the anoxic mix zone into the aerobicsequencing batch aeration treatment zone until a predetermined upperliquid level is reached in the aerobic sequencing batch reaction zone;

an interaction step in which the waste liquid is aerated or mixed in theaerobic sequencing batch aeration treatment zone while liquid from theaerobic zone containing nitrate or nitrite components is introduced intothe anoxic mix zone, and anoxic waste liquid from the anoxic zone isintroduced into the aerobic zone. Preferably, this introduction ofliquid from the aeration treatment zone into the anoxic mix zone, and ofliquid from the anoxic mix zone into the aerobic treatment zone are eachcarried out at a rate of at least 0.2 times the volume of liquid in theanoxic mix zone per hour, and more preferably, at a rate in the range offrom about 50 to about 500 percent of the total volume of waste liquidin the aerobic treatment zone, per hour, during the interaction step.

a settling step in which influent wastewater is introduced into theanoxic treatment zone while the waste liquid in the aerobic zone ismaintained in a quiescent condition without substantial mixing, aerationor introduction of wastewater from the anoxic zone to provide an upperzone of clarified wastewater, and a decantation step in which clarifiedeffluent is withdrawn from the clarified upper zone of the aerobictreatment zone.

The cycle times for the individual steps and the overall batch processcycle time will depend on a variety of system design parameters, as wellas the wastewater impurity loading, the water temperature and similarfactors. The initiation of each step, and the conclusion of a precedingstep may be controlled in any suitable manner, such as on a timed basis,or may be triggered by various water level conditions in the treatmentsystem. The timing of the sequential processing steps may also becontrolled by appropriate sensors such as oxygen and nitrate sensors.The total treatment cycle may typically take from about 2 to about 12hours, but may be longer, for example, under cold weather or highorganic or nitrogen loading conditions. The fill step is typicallycarried out for about 10% to about 30% of the total treatment cycletime, which may typically be from about 6 minutes or 0.1 hours to about60 minutes or 1 hour, for a system having a hydraulic retention time offrom about 16 to about 20 hours. The interaction step is typicallycarried out for about 25% to about 75% of the total treatment cycletime, which may typically be from about 0.1 hours to about 2 hours, fora system having a hydraulic retention time of from about 16 to about 20hours. The settling step is typically carried out for about 10% to about30% of the total treatment cycle time, which may typically be from aboutto about 0.5 to 1 hours, for a system having a hydraulic retention timeof from about 16 to about 20 hours. The decantation step is typicallycarried out for about 5% to about 25% of the total treatment cycle time,which may typically be from about to about 0.1 to 1 hours, for a systemhaving a hydraulic retention time of about 16-20 hours.

The methods may further include a separate aeration and/or mixingreaction step after the interaction step and before the settling step,in which the influent is introduced into the anoxic mixing zone, and thewastewater in the aerobic treatment zone is mixed and aerated withoutintroduction of wastewater from the anoxic mix zone into the aerobictreatment zone. The optional separate aeration and/or mixing step maytypically be carried out from about 0 to about 12 hours, preferably fromabout 0.1 to about 3 hours, or until the BOD is reduced to apredetermined value such as less than 20 mg/l.

In such treatment methods, the wastewater solids, particularly includingthe microbiological treatment organisms which grow in the wastewater maybe periodically removed from the treatment system. In this regard, aportion of wastewater solids may desirably be removed from the aerobictreatment zone during or after decantation of clarified effluent, andbefore completion of the filling step. The wastewater containingwastewater microorganisms and other solids may be pumped to conventionalsolids separation, digestion or disposal apparatus in accordance withconventional practice. However, a portion of the wastewater containingsuch solids may also be introduced into an anaerobic digestion zone,such as an anaerobic “trash trap” for subsequent reintroduction into theanoxic mix zone. In this manner, the total solids produced by thetreatment system may be significantly reduced, by anaerobic digestion ofthe waste solids. In addition, the removal of phosphorous may befacilitated. In this regard, when mixed liquor microorganisms aresubjected to anaerobic conditions, the cells tend to give up phosphorousand ammonia back into solution, to create a more phosphorous andnitrogen-rich influent stream. When the surviving microorganisms aresubsequently subjected to an anoxic or aerobic environment in the anoxicmix or aerobic sequencing batch reaction zones, they tend to take upmore phosphorous than was given up, particularly in respect tophosphotroph metabolism.

Because the settle and decant cycles are time-consuming, anoxicwastewater may be sequentially transferred from the anoxic mix zone to aplurality of aerobic treatment zones, in order to maximize theutilization of the anoxic mix zone. In this regard, the anoxic mix zonemay fill and interact with a second aerobic zone while the first aerobictreatment zone is in its settle and decant cycles. This optimizes theanoxic mix zone utilization, because it has a more constant supply ofnitrate and nitrite nutrient for its operation, and reduces theequalization or “buffering” tank volume required to accommodate thecontinuous influent flow during the sequential treatment cycle of thesystem. This can also reduce the change in surge anoxic mix treatmentzone operating level during the treatment cycle, and the lift level forpumping to the aeration treatment zone, which increases energyefficiency.

The present invention is also directed to multi-chamber sequencing batchreactor systems for wastewater treatment. Such treatment systemsgenerally comprise a surge anoxic mix reaction tank, an anoxic reactiontank inlet for introducing wastewater to be treated into the anoxicreaction tank, and an aeration reaction tank for mixing and aeratingwastewater and wastewater treatment microorganisms, and a pump forintroducing waste liquid from the surge anoxic mix reaction tank to theaeration reaction tank. The aeration reaction tank will include anaerator for aerating wastewater in the aerobic reaction tank and adecanter for withdrawing liquid from the top of the aerobic reactiontank. An important component of the treatment system is a means forintroducing wastewater from the aerobic reaction tank to the anoxic mixtreatment tank, preferably while wastewater from the anoxic mix tank ispumped into the aeration reaction tank. The means for transferringwastewater from the anoxic mix tank to the sequencing aeration tank isdesirably a pump such as an air lift pump or a centrifugal pump or pumpshaving a total pumping capacity of at least about 2 and preferably atleast 3 times the average daily design treatment flow capacity of thewastewater treatment system, and the means for introducing wastewaterfrom the aerobic reaction tank into the anoxic reaction tank ispreferably a passive overflow design having an intake orifice or weir ata predetermined top liquid level in the aeration reaction tank and whichdischarges into the anoxic reaction tank. Separate pumping systems maybe used to respectively pump from the anoxic mix zone to the aerobiczone, and from the aerobic zone to the anoxic mix zone. However, the useof a passive overflow weir is particularly efficient and maintenancefree, and advantageously returns surface scum to the anoxic tank forfurther processing. In addition, it accommodates continuous flow withoutvalving between tanks, and still allows for quiescent settling.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with method aspects of various embodiments of the presentinvention, wastewater to be treated may be introduced sequentially intointeracting mixed liquor treatment zones containing waste solids,including treatment microorganisms. The aerobic wastewater mixed liquoris aerated under predetermined conditions for a period of time to reducethe biological oxygen demand of the wastewater. The mixed liquor may besubsequently maintained in a quiescent state in the sequencing batchaeration zone for a settling period to form a clarified upper layer anda stratified lower layer containing waste solids. For many systems,typically the mixing and aeration time period (fill and interact) may bein the range of from about 1 hour to about 5 hours, and preferably fromabout 2 to about 4 hours. The settling, quiescent period will typicallybe less than about 2 hours, and preferably in the range of from about 30minutes to about 90 minutes. The decantation time will typically be inthe range of from about 0.1 to about 1 hour. These times are pertreatment cycle.

Illustrated in FIG. 1 is a schematic process flow diagram of anembodiment of a surge anoxic mix, sequencing batch reaction process inaccordance with the present invention, which illustrates a mode ofoperation of the wastewater treatment system shown in FIGS. 3, 4 and 5.

As shown in FIG. 1, in accordance with various embodiments of thewastewater treatment methods of the present invention, an influentstream 100 of wastewater to be treated is continuously introduced into asurge anoxic mix treatment zone 102 which is adjacent to an aeratedsequencing batch reaction zone 104. The wastewater 8 to be treated maytypically be municipal wastewater, which is generated continuously,although the flow rate may vary both seasonally, and over the course ofeach day or week. The influent stream may be introduced directly fromthe municipal, industrial or other generating source, but will typicallybe pretreated by grit and solids removal systems, and may be firstintroduced into an anaerobic treatment zone, such as a “trash trap”,before introduction into the anoxic mix zone. In some embodiments of thepresent invention, the anaerobic treatment zone may be used to furtherreduce solids produced by the system, as will be described.

In FIG. 1, the treatment method is illustrated in five treatment phases:an initial fill phase, an interaction phase, an optional react phase, aclarification phase, and a treated effluent removal phase. In the fillphase, the aeration treatment zone is filled with wastewater from theanoxic mix zone, as influent wastewater to be treated is introduced intothe anoxic mix zone. The introduction of the wastewater into the aeratedbatch reactor zone 104 is begun at a relatively low liquid level in theaerated zone 104. The surge anoxic mix zone 102 and the sequencing batch(aeration) reaction zone 104 contain mixed liquor including retained,“activated sludge” treatment microorganisms. The surge anoxic mix zone102 may be mechanically or hydraulically mixed, but is generally notfully aerated, such that it is in an anoxic condition conducive toutilization of nitrates and nitrites as oxidizing agents by the mixedliquor bacterial cultures in the zone. The sequencing batch reactionzone 104 is relatively highly aerated, and is also mixed eithermechanically or hydraulically, such that the mixed liquor in the zone isin an aerated condition conducive to the utilization of oxygen as theoxidizing agent by the mixed liquor cultures in the aerated zone 104.The fill phase is continued until the aerated reaction zone is filled toa predetermined height or volume. Preferably, this height will bedetermined by the intake height of a passive liquid return weir andconduit for returning liquid to the anoxic mix zone. The fill step maybe, or is, terminated when the aerobic SBR tank 104 is full, such thatnitrate-containing wastewater from the aerobic tank zone 104 is returnedto the anoxic mix zone 102.

The interaction phase follows the fill phase. During the interactionphase, the mixed liquor undergoing aerobic oxidation treatment in theSBR zone is intermixed with the mixed liquor and influent 100 undergoinganoxic treatment in the anoxic treatment zone 102, while the influentwastewater 100 is introduced into the anoxic zone. For every gallonpumped from the anoxic tank, one gallon returns from the aerobic tank.The interact step may include intermittent pumping and aerationcontrolled by time or dissolved oxygen and/or nitrate sensors. It isnoted that filling of the treatment system (zone 102) also continuesthroughout the interact step, because the influent wastewater to betreated continues to flow into the anoxic treatment zone.

Any suitable pump or pumps may be utilized, such as electricmotor-driven fluid pumps, or air-lift pumps. During the interactionphase or step, anoxic mixed liquor from the surge anoxic mix zone 102 isintroduced into the aerated batch reaction zone 104, and aerated mixedliquor from the sequencing batch reaction zone 104 is introduced intothe surge anoxic mix zone. This may be accomplished by pumping mixedliquor from the anoxic mix zone into a substantially full aeratedreactor zone 102, and permitting the mixed liquor from the aerated batchreaction zone 104 to overflow back into the surge anoxic mix zone. Apassive overflow system is relatively maintenance-free and energyefficient, and has other advantages such as returning any floating scumor debris to the anoxic mix zone. The pump rate (or intermixing rate)should be at least two times the average daily rate of introduction ofinfluent wastewater 100 into the surge anoxic mix zone, and preferablywill be at least 5 times the average daily rate of introduction ofinfluent wastewater 100 into the zone 102.

It will be appreciated that in the surge anoxic mix zone, influentwastewater components, including organic components constituting BOD,are partially consumed by mixed liquor microorganisms using nitrates andnitrites produced in the aerated batch reaction zone which aretransferred and mixed from the aerated zone 104 into the zone 102.Further, nitrate and nitrite components are reduced to nitrogen gas, andthereby removed from the wastewater. The anoxic mix zone has a reactiongradient of relatively high BOD and low dissolved oxygen whichfacilitates denitrification, while the aerobic treatment zone has areaction gradient of relatively low BOD and high dissolved oxygen, whichis more optimal for nitrification along with BOD removal. Accordingly,during the interaction phase, the wastewater components are also rapidlyoxidized in the aerated batch reaction zone 104, which also producesnitrates and nitrites in the mixed liquor for utilization (and removalas N₂) in the surge anoxic mix zone. The interaction phase will usuallyconstitute from about 20% to about 80% of the total cycle time of thesequencing batch reaction process.

Waste solids (sludge) may be removed from the anoxic zone during theinteraction phase of the treatment cycle, or may be introduced into ananaerobic pretreatment zone, as previously discussed, for digestion andsubsequent reintroduction into the anoxic mix zone to facilitate totalsolids reduction and/or phosphorous removal. The mixed liquor in theanoxic zone may be continuously or intermittently mixed in the anoxiczone during the interact phase in an appropriate manner, such as by jetmotive pumps or by aerobic zone recycle, but will generally not beaerated (except in unusual circumstances such as influent flowconditions exceeding the system design conditions). The rate at whichthe mixed liquor from the anoxic mix zone is introduced into the aeratedSBR zone will desirably be at least 2 and preferably at least 3 timesthe average influent flow rate of the influent wastewater 102, and inhighly effective embodiments will be at least about 5 times the influentwastewater flow rate. In this regard, the pumping capacity should bestexceed the peak design influent flow rate, which may typically be fromabout 2 to 4 times the average influent flow rate for municipalwastewater treatment systems.

The mixed liquor in the SBR zone 104 is continuously or intermittentlyaerated and mixed within the treatment zone 104, in order to efficientlyand effectively foster biooxidation of the wastewater in the zone.Continuous aeration and mixing produced by motive jet aerators such asF2JA jet aerators manufactured by Fluidyne Corporation having anoxygenation transfer efficiency of at least 20% are effective for mixingand aeration in an energy efficient manner. In systems in which theprogression of the treatment phase is determined by water levelconditions, the interact phase may extend from the time the aerobicsequential batch aeration (SBR) zone 104 is filled (and overflows backinto the anoxic zone 102), until the liquor level in the anoxic zonereaches a control water level set point, or an intermediate level if anoptional aeration react phase is utilized. Alternatively, a timedtreatment control system and/or nitrate and oxygen sensors may be usedto control the length of the interaction phase.

As indicated, following the interaction phase, either an optionalaeration reaction phase, or a clarification phase is carried out. Anaeration reaction phase, even of relatively short duration, may bedesirable to insure that at least the most readily biooxidizablecomponents of the wastewater, particularly that which was most recentlyintroduced from the anoxic treatment zone, has been treated in the SBRzone for a suitable period of time without introduction of “fresh”wastewater from the anoxic zone. During the SBR aeration reaction phase,the influent wastewater continues to be introduced into the anoxic zone,which serves as a volume buffer as it fills. The anoxic zone may bemixed or stirred, but wastewater is generally not introduced insignificant amounts from the anoxic zone 102 into the SBR zone 104during the optional reaction phase, if such a phase is utilized.

Following the reaction phase, or the interaction phase if no reactionphase is utilized, a clarification step is carried out in which themixing and aeration in the SBR zone 104 is stopped, so that it becomesquiescent for settling of the microbiological treatment cultures toprovide a clarified upper layer. The time to achieve effectiveclarification will typically be about 45 minutes. It is noted thatduring the clarification phase in the SBR zone 104, the influentwastewater 100 may continue to be introduced into the anoxic treatmentzone 102, raising the liquid level in the zone 102. The clarificationand settling phase is continued for a predetermined amount of timesufficient to allow biological solids to settle well below the bottomwater level achieved in the SBR zone 104 after decant.

In this regard, after the mixed liquor in the SBR zone 104 has settledto provide a clarified upper zone 106 and a lower sludge zone 108, thetreated, clarified effluent in the zone 106 is removed, preferably to apredetermined level, without substantially remixing the settled sludge.Decanting systems such as the fixed, air operating solids excludingdecanter (SED) decanting systems described in U.S. Pat. No. 4,596,658and manufactured by Fluidyne Corporation may be used to decant surfacewater without substantial turbulence. Typically, at least about 15% ofthe water in the SBR zone will be decanted in the effluent removalphase. During the effluent removal phase, the influent wastewater maycontinue to be introduced into the anoxic mix zone 102, raising thewater level to its maximum height. Without the intermixing ofnitrate-containing mixed liquor from the SBR zone, the mixed liquor inthe zone 102 may become anaerobic if left too long, and accordingly maybe mixed or moderately aerated if appropriate.

Following the effluent removal phase (decant), the treatment cycle isrepeated. The aerobic SBR zone 104 is filled in a fill phase by pumpingmixed liquor from the filled anoxic mix zone 102 into the SBR zone 104,which is at its low water level following clarified effluent removal,and the remaining steps are repeated.

Such sequencing batch reactor systems of surge anoxic mix interactiondesign having an anoxic mix chamber interacting with a sequencing batchaeration and decantation chamber can provide significant advantages overconventional sequencing batch reactors having a single sequencing batchaeration and decantation processing chamber. The following calculateddesign comparison of a conventional sequencing batch reactor system anda sequencing batch reactor system of interacting surge anoxic mix designillustrates some of the potential advantages. The comparison shows totaltank volume reduced to 71% of the conventional SBR volume (1.08 mg vs0.77 mg), and operating power requirements reduced from 56 bhp to 44bhp:

TABLE 1 Conventional Aerobic SBR Sequencing with Interacting BatchReactor Surge Anoxic System Mix Zone INFLUENT CONDITIONS Flow (m3/d)3785 3785 Flow (mgd) 1.000 1.000 Flow (gpm) 694 694 BOD (mg/l) 200 200BOD (lb/d) 1668 1668 TSS (mg/l) 200 200 TSS (lb/d) 1668 1668 NH3-N(mg/l) 30 30 NH3-N (lb/d) 250 250 OXYGEN REQUIREMENTS Lbs. TKN requiredfor synthesis 58 58 Lbs. NO3-N produced 192 192 Lbs. 02 recovered/lbNO3-N reduced 2.6 Lbs. Oxygen/lb. Of BOD 1.4 1.4 Lbs. Oxygen/lb. TKN 4.64.6 Actual Oxygen Demand (lb 02/d) 3486 2719 Total Alpha 0.9 0.9 Beta0.95 0.95 Theta 1.024 1.024 Operating Dissolved Oxygen (mg/l) 1 1 CleanWater Oxygen Sat. at Op. 9.09 9.09 Temp (Mg/l) Clean Water oxygen sat.at Std. 9.09 9.09 Temp (mg/l) Clean Water 02 sat, std temp, 11.50 11.50mid depth (mg/l) Std. Condition ambient pressure 14.7 14.7 (psia) Oper.Condition ambient pressure 14.5 14.5 (psia) Wastewater temperature (c)20 20 SOR/AOR ratio 1.31 1.31 Standard Oxygen Demand (lb 02/d) 4557 3554total Standard Oxygen Demand (lb/02/hr) 380 296 Specific oxygenationrate (mg/l-hr) 42 70 Lbs. of oxygen/lb. Of air 0.23 0.23 Clean WaterEfficiency (%) 25 25 Lbs. of Air/Cubic Ft. of air 0.075 0.075 Aerationhours per day 12.00 12.00 Air flow rate (SCFM/tank) 734 572NITRIFICATION/DENITRIFICATION Required alkalinity for nitrifi- 164 164cation (mg/l) Alkalinity recovered, denitrifi- 69 69 cation (mg/l) Netalkalinity required (mg/l) 95 95 Mixed liquor temperature, C. 15 15 MLdissolved oxygen (mg/l) 1 1 Max. nitrifier growth rate, day-1 0.2040.204 Minimum solids retention time (SRT) 4.89 4.89 required fornitrification, days Actual or Design SRT, days 19.27 8.68 Kn, halfvelocity constant (mg/l) 0.40 0.40 Design growth rate for hetero- 0.05190.1152 trophs/nitrifiers Projected effluent soluble NH3-N, 0.14 0.52mg/l Specific utilization rate, lbs 0.19 0.30 BOD5/lb mlvss Lbs. mlvssrequired for BOD & 9002 5552 NH3 removal mlvss (mg/l) 1500 1500 Tankvolume req. for BOD & NH3 0.72 0.44 removal (MG) Denitrification rate(g/g/day) 0.043 0.047 lbs mlvss required for denitrifi- 4484 4076 cationTank volume required for NO3 0.36 0.33 removal (MG) SBR TankConfiguration No. of tanks 2 2 Length SBR (ft) 90 42 Length Surge anoxictank (ft) 27 Width (ft) 45 45 Bottom water level (ft) 14 14 Top waterlevel (ft) 18 18 Cycle Water Level 15 No. decanters/tank 4 4 SBR tankagevolume a TWL (MG) 1.0906 0.5089 HRT (hrs) 26.17 12.21 CYCLETIMES/CAPACITY CALCULATIONS Total Decant Volume (cubic feet) 32,40015,120 Total Decant Volume (gallons) 242,352 113,098 Decant volume pertank (gallons) 121,176 56,549 Number of cycles per day/tank 4.13 8.84Total time per cycle (minutes) 349 163 Fill rate (gpm) 694 2730 Filltime (minutes) 174 21 Feed rate (gpm) 694 347 Interact period (minutes)81 React period (minutes) 69 Settle period (minutes) 45 45 Averagedecant rate (gpm/ft 100 100 decanter) Decanter length (feet) 36 36Decanting time (minutes) 34 16 Decanting rate (gpm) 3600 3600 Peakdecanting rate (gpm at start 3960 3960 of decant) Idle period time(minutes) 27 0 Zero idle & react time, flow rate 2.22 2.00 (MGD) Maximumaeration period available 13.85 15.05 (hours/day) EQUIPMENT SELECTIONAir flow per nozzle (scfm) 38 38 Number of nozzles required (per 19.3115.06 tank) Number of nozzles provided (per 20 16 tank) Actual airflowper nozzle re- 36.69 35.77 quired (scfm) Blower capacity provided (scfm)734 572 POWER CONSUMPTION CALCULATIONS Pump Efficiency 0.76 0.76 BlowerEfficiency 0.6 0.6 Pump horsepower, BHP/tank 24 19 Mixing BHP/MG 32 25Blower horsepower/BHP/tank 58 44 Total horsepower, BHP/tank 56 44Aeration BHP/MG 103 174 Total design equivalent horsepower, 56 44 BHPSLUDGE PRODUCTION Sludge Yield Factor 0.7 0.7 Net Sludge Yield (lbs/d)1012 1048 Sludge Concentration (%) from SBR 0.21 0.21 Sludge WastingRate (gpd) 56609 58644 Waste Sludge/cycle (gal) 6860 3316 WAS PumpingRate (gpm) 75 75 Waste Sludge Cycle Time (min) 91.5 44.2 ThickenedSludge Concentration (%) 1.5 1.5 Thickened Sludge (gpd) 8087 8378 MLSS(mg/10 @ TWL 2143 2143 Sludge Inventory Total (lbs) 19490 14943 SludgeInventory in SBR (lbs) 19490 9595 SRT (1/days) Total 19.27 14.26 SRT inSBR (1/days) 19.27 8.68 F/M 0.09 0.11 SV1 (ml/g) 200 200 Sludge blanketlevel (ft) 7.73 7.73 Organic loading (lbs (BOD/1000 ft3) 11.44 14.92

As discussed in connection with the treatment methods embodimentillustrated in FIG. 1, there is little or no interaction between thewastewater and microbial treatment cultures in the surge anoxic mixzone, and the SBR zone during clarification phase and the optional reactphase of the treatment cycle. Thus, the introduction of nitrate/nitritecomponents which serve as oxidation energy source is cyclical to thesurge anoxic mix zone in the methods of FIG. 1. Depending on factorssuch as the relative amount of BOD and nitrogen to be removed from theinfluent wastewater, and the storage volumes appropriate to accommodatethe influent wastewater during the various sequencing treatment steps,it may be desirable to provide more continuous interaction between thesurge anoxic mix zone and an aerated SBR zone. In this regard,schematically illustrated in FIG. 2 is an alternative embodiment of awastewater treatment method similar to that of FIG. 1, but in which asurge anoxic mix zone 202 interacts with a plurality of (here, two)aerated sequencing batch reaction zones 104, 106 having staggeredtreatment cycles such that fill and interaction phases of one aerationzone correspond to the settle and decant phases of the other aerationzone. As shown in FIG. 2, the surge anoxic mix zone 202 has a doublecycle which alternates between the two SBR zones 204, 206, so that it issubstantially continuously filling and/or interacting with at least oneof the SBR zones 204, 206 during the quiescent and decant phases of theother. The size of anoxic vs. aerobic treatment zones is a function ofreaction treatment by kinetics as well as hydraulic requirements. Thehydraulic considerations of feeding two aerobic treatment zones with onesurge anoxic mix zone permits a smaller anoxic zone to total aerobiczone volume-ratio, or conversely less draw down in the surge anoxic mixzone for the same volume ratio. Typically, the biokinetics will be usedto calculate the tank sizing, which may then be adjusted for hydraulicrequirements of the influent flow.

As shown in FIG. 2, the “first” of the surge anoxic mix cycles isinitiated with one of the SBR zones 204. Then, during the (optional)react phase, clarification phase and effluent removal phase of the SBRtreatment zone 204 during which there is no substantial interactionbetween the surge anoxic mix zone 202 and the SBR zone 204, a “second”interaction cycle is initiated between the surge anoxic mix zone 202 andthe second SBR zone 206. As will be appreciated from FIG. 2, byoffsetting the cycles of the two aeration and decantation zones, theanoxic mix zone may be more effectively utilized. Such a method mayprovide enhanced biological efficiency in the surge anoxic mix zone, andcost improvements related to tankage volume utilization improvementsbecause less surge anoxic mix storage capacity may be needed toaccommodate a continuous wastewater influent flow for a given SBR cycletime. Operational advantages include less anoxic mix level drawn down,more consistency in NO₃ return and mixing without auxiliary mixers, andbetter hydraulic peak flow handling. Disadvantages include redundancyand turn down. Particularly in view of the high capacity of dual feedmethods and treatment systems like that of FIG. 2, and particularly atlow influent flow conditions, there may be an “idle” time periodfollowing the decant step in the respective aerobic zones before thereis sufficient influent to being a new full step.

As indicated previously, the present invention is also directed tomulti-chamber sequencing batch reactors for wastewater treatment whichare designed to utilize interacting anoxic and aerated aerobicsequencing batch reaction tanks. Such SBR systems comprise an anoxicreaction tank having an inlet for introducing wastewater to be treatedinto the anoxic reaction tank, sequencing batch aerobic reaction tank(SBR) for receiving wastewater from the anoxic reaction tank and forsequentially mixing and aerating wastewater and wastewater treatmentmicroorganisms, settling the wastewater, and withdrawing clarified waterfrom the top of the aerobic reaction tank and for introducing wastewaterfrom the aerobic reaction tank to the anoxic mix treatment tank.

The sequencing batch reaction system will desirably include a pump fortransferring wastewater from the anoxic mix tank to the sequencingaerobic batch reaction tank having a pumping capacity of at least about0.2, and preferably from about 0.5 to 5 times the anoxic mix tank volumeper hour and a passive overflow weir at a predetermined or adjustabletop liquid level in the aerobic reaction tank for introducing wastewaterfrom the aerobic reaction tank into the anoxic reaction tank. Thesystems may include a bottom water level control system for the aerobicreaction tank, which initiates a system fill cycle when triggered by thewater level in the aerobic SBR tank reaching the bottom water level(BWL). The control system operates to start feed and/or jet motive pumpto pump from the surge anoxic mix tank to the aerobic reaction tank.After the SBR tank is full and after predetermined (adjustable) timeperiod after fill or until a dissolved oxygen set point is reached, thecontrol system operates to continue to cycle feed and/or jet motive pumpbased on time and/or D.O. and/or NO₃ levels on a continuous orintermittent basis during the interaction phase, as previouslydescribed. The control system also includes a control water level (CWL)sensor in the surge anoxic mix tank. The control system operates suchthat when the surge anoxic mix tank wastewater level reaches CWL, thecontrol water level in the surge anoxic mix tank, the feed pump and/orjet motive pump is stopped and a settle timer is started for theclarification cycle. After the settle timer times out, start decant. TheSBR level reaches bottom decant level BWL, start feed and/or jet motivepump (or waste sludge pump first).

Illustrated in FIGS. 3-5 is an embodiment 300 of a surge anoxic mix,sequencing batch reactor municipal wastewater treatment plant designedfor a nominal peak treatment capacity of 0.5 to 4.4 mgd. The system hasa relatively high ratio of peak hydraulic flow to average influent flow,demonstrating the versatility of the system (it is noted that at peakflow, the system does not operate to separately produce NO₃. FIG. 3 is aplan view of the system 300, which has two separate, and substantiallyidentical, treatment systems 302, 304 (phase 1 and phase 2). The designspecifications for the treatment system 300 are set forth in thefollowing Table 2 for the average daily wastewater (ADW) design flowrate, and the peak design flow rate, with specification and terms asdefined in Mandt, et al. supra:

TABLE 2 AVERAGE AVERAGE DAILY WASTE PEAK DAILY WASTE WATER HYDRAULICWATER PEAK PEAK FLOW FLOW FLOW HYDRAULIC FLOW METRIC PHASE 1 PHASE 1PHASE 2 PHASE 2 PHASE Influent Conditions Flow (m3/d) 2534 8697 462616430 Flow (mgd) 0.669 2.298 1.222 4.351 Flow (gpm) 465 1596 849 3014190 lps BOD (mg/l) 161 94 147 83 BOD (lb/d) 900 1800 1503 3005 1366 kg/dTSS (mg/l) 116 68 121 68 TSS (lb/d) 647 1294 1232 2464 1120 kg/d NH3-N(mg/l) 30 17 30 17 NH3-N (lb/d) 168 335 306 612 278 kg/d OXYGENREQUIREMENTS Lbs. TKN required 31 63 53 105 for synthesis Lbs. NO3-N 1360 253 0 produced Lbs. 02 2.6 0.0 2.6 0.0 recovered/lb NO3- N reducedLbs. Oxygen/lb. of 1.4 1 1.4 1 BOD Lbs. Oxygen/lb. 4.6 0 4.6 0 TKNActual Oxygen 1532 1800 2610 3005 1366 Demand kg/d (lb 02/d) Total Alpha0.9 0.9 0.9 0.9 Beta 0.95 0.95 0.95 0.95 Theta 1.024 1.024 1.024 1.024Operating 2 0.5 2 1 Dissolved Oxygen (mg/l) Clean Water 9.09 9.09 9.099.09 Oxygen Sat. at Op. Temp (Mg/l) Clean Water 9.09 9.09 9.09 9.09Oxygen Sat. at Std. Temp (mg/l) Clean Water 02 Sat, 11.50 11.50 11.5011.50 Std Temp, Mid Depth (mg/l) Std. Condition 14.7 14.7 14.7 14.7Ambient Pressure (psia) Oper. Condition 14.5 14.5 14.5 14.5 AmbientPressure (psia) Wastewater 20 20 20 20 Temperature (c) SOR/AOR ratio1.46 1.24 1.46 1.31 Standard Oxygen 2230 2238 3800 3928 Demand (lb 02/d)total Standard Oxygen 161 230 300 391 Demand (lb/02/hr) Specific 33 4730 40 Oxygenation Rate (mg/l-hr) Lbs. of Oxygen/Lb. 0.23 0.23 0.23 0.23of Air Clean Water 25 25 25 25 Efficiency (%) Lbs. of Air/Cubic 0.0750.075 0.075 0.075 Ft. of Air Aeration Hours Per 13.88 13.88 13.88 13.88Day Air Flow Rate 311 445 290 378 (SCFM/tank) Air Pressure Losses 0.70.7 0.7 0.7 (lines and nozzle) Maximum Air 7.64 7.64 7.64 7.64 Pressure(psig) Average Air 6.72 6.72 6.72 6.72 Pressure (psig) NITRIFICATION/DENITRIFICATION Required alkalinity 174 0 177 0 for nitrification (mg/l)Alkalinity 73 0 75 0 recovered, denitrification (mg/l) Net alkalinity101 0 103 0 required (mg/l) Mixed liquor 20 20 20 20 temperature, C. MLdissolved 1 1 1 1 oxygen (mg/l) Max. nitrifier 0.334 0.334 0.334 0.334growth rate, day-1 Minimum SRT 3.00 3.00 3.00 3.00 required fornitrification, days Kn, half velocity 0.73 0.73 0.73 0.73 constant(mg/l) Design growth rate 0.0366 0.0813 0.0293 0.9665 for heterotrophs/nitrifiers Projected effluent 0.09 0.23 0.07 0.18 soluble NH3-N, mg/lSpecific utilization 0.16 0.24 0.14 0.21 rate, lbs BOD5/lb mlvss Lbs.mlvss required 5713 7541 10427 14185 for BOD & NH3 removal mlvss (mg/l)2000 2000 2000 2000 Tank volume req. 0.34 0.45 0.63 0.85 for BOD & NH3removal (MG) Aerobic hrs/day 13.88 18.32 12.66 17.23 required, hr.Denitrification rate 0.060 0.060 0.060 0.060 (g/g/day) Lbs mlvssrequired 2267 0 4220 0 for denitrification Tank volume 0.14 0/00 0.250.00 required for NO3 removal (MG) Anoxic hrs/d 5.51 0.00 5.13 0.00required/hr. Total tank volume 0.48 0.45 0.88 0.85 required (MG) SBRTANK CONFIGURATION No. of tanks 2 2 4 4 Length (ft) 55.76 55.76 55.7655.76 17 Width (ft) 39.36 39.36 39.36 39.36 12 Bottom water level 13.77613.776 13.776 13.776 4.2 (ft) Top water level (ft) 18.04 18.04 18.0418.04 5.5 No. decanters/tank 2 2 2 2 Total Tankage 0.5923 0.5923 1.18461.1846 Volume @ TWL (MG) HRT (hrs) 21.23 6.19 23.26 6.55 CYCLE TIMES/CAPACITY CALCULATIONS Total Decant 18,717 18,717 37,433 37,433 1061Volume (cubic feet) M3 Total Decant 140,000 140,000 279,999 279,999Volume (gallons) Decant volume per 70,000 70,000 70,000 70,000 265.2tank (gallons) M3/tank Number of cycles 4.78 13.29 4.36 12.99 perday/tank Total time per cycle 301 108 330 111 (minutes) Fill rate (gpm)2389 2389 2389 2389 Fill time (minutes) 29 29 29 29 Fill time surge 12115 53 17 anoxic mix tank (minutes) SWL Interact period 121 15 53 17(min) Settle period 45 45 45 45 (minutes) Average decant rate 100 100100 100 (gpm/ft decanter) Decanter length 36 36 36 36 (feet) Decantingtime 19 19 19 19 (minutes) Decanting rate 3600 3600 3600 3600 (gpm) Peakdecanting rate 3960 3960 3960 3960 (gpm at start of decant) Idle periodtime 86 0 183 0 (minutes) Zero idle & react 1.57 1.57 1.57 1.57 time,flow rate (MGD) Maximum aeration 18.86 9.72 19.31 10.05 period available(hours/day) EQUIPMENT SELECTION Air flow per nozzle 35 35 35 35 (scfm)Number of nozzles 8.87 12.71 8.28 10.79 required (per tank) Number ofnozzles 14 14 14 14 provided (per tank) Actual airflow per 22.18 31/7820.71 26.97 nozzle required (scfm) Blower capacity 311 445 290 378provided (scfm) POWER CONSUMPTION CALCULATIONS Pump efficiency 0.73 0.730.73 0.73 Blower efficiency 0.6 0.6 0.6 0.6 Pump horsepower, 14 14 14 14BHP/tank Mixing BHP/MG 49 49 49 49 Blower horsepower, 14 20 13 17BHP/tank Total horsepower, 28 34 27 31 BHP/tank Aeration BHP/MG 95 11592 105 Total design 39 28 70 52 equivalent horsepower, BHP SLUDGEPRODUCTION Sludge Yield Factor 0.7 0.7 0.7 0.7 Net Sludge Yield 517 1147826 1878 (lbs/d) Sludge 0.29 0.29 0.29 0.29 Concentration (%) from SBRSludge Wasting 21695 48139 34664 78805 Rate (gpd) Waste Sludge/cycle2268 1810 1985 1517 (gal) WAS Pumping 50 50 50 50 Rate (gpm) WasteSludge Cycle 45.4 36.2 29.7 30.3 Time (min) Thickened Sludge 1.5 1.5 1.51.5 Concentration (%) Thickened Sludge 4132 9169 6603 15011 (gpd) MLSS(mg/l) @ 2857 2867 2867 2867 TWL Sludge inventory 14114 14114 2822828228 (lbs) SRT (1/days) 27.30 12.30 34.17 15.03 F/M 0.06 0.13 0.05 0.11SVI (ml/g) 200 200 200 200 Sludge blanket 10.32 10.32 10.32 10.32 level(ft) Organic loading 11.36 22.73 9.49 18.98 (lbs BOD/1000 ft3) AEROBICDIGESTER Number of tanks 1 1 2 2 Length (ft) 55.76 55.76 55.76 55.76 17Width (ft) 39.36 39.36 39.36 39.36 12 TWL (ft) 18.04 18.04 18.04 18.045.5 Total volume (gal) 296,153 296,153 592,306 592,306 available Dayssludge storage 71.67 32.30 89.71 39.46 available Total sludge age 98.9744.60 123.88 54.49 including SBR (days) Pounds sludge 265 318 491 615destroyed % sludge reduction 51 28 59 33 Thickened, digested 2011 66282680 10096 sludge (gpd) Lbs oxygen/sludge 1.42 1.42 1.42 1.42 destroyedAeration hours/day 10 10 10 10 SOR/tank (lbs/hr) 57 68 52 65 Clean water25 25 25 25 efficiency (%) SCFM/tank 218 262 202 253 Air flow per jet 3535 35 35 Number of jets 6.24 7.48 5.77 7.23 required per tank Number ofjets 14 14 14 14 provided Pump horsepower 14.40 14.40 14.40 14.40 (bhp)Blower horsepower 11 14 11 13 (bhp) Mixing energy 87 95 84 93 (hp/MG)Total design 11 12 21 23 equivalent horsepower, BHP

In the operation of the treatment plant 300, the influent wastewater 306is introduced into an influent sieve tank 308 containing a spiral sieve310 of conventional design to remove solid wastewater components. Thesieved wastewater flows through manually or automatically controlledstop gate 312 and sluice gate 314, into a generally anaerobic grease andgrit trap tank 316, which includes a sludge holding and thickening tank318, a jet aspirator 320 to permit control of anaerobic conditiondevelopment, and tank baffles 322, in accordance with conventionaldesign practice. The wastewater from the grit and grease trap tank 316is introduced into the surge anoxic mix tank 324 through conduit 326,which positions the influent wastewater toward the bottom of the tank324 for odor control. The surge anoxic mix tank 324 is also providedwith a linear array of motive jet mixers 326 for mixing the liquid andtreatment cultures in the tank. The jet mixers are powered by a 15horsepower motive jet pump 328 having an intake within the surge anoxicmix tank 324. The mixed liquor wastewater undergoing treatment in thesurge anoxic mix tank 324 may be pumped to either or both of thesequencing batch reaction tanks 330, 332. As best shown in FIG. 5, whichis a cross-sectional view of the surge anoxic mix tank 324 and thesequencing batch reaction tank 330, a motive pump 332 with an intakepositioned within the surge anoxic mix tank 324 is utilized to power alinear array 336 of jet aerators, such as model F2JA jet aerators ofFluidyne Corporation positioned within the sequencing batch reactiontank. Pressurized air is also provided to the jet aerators by means ofblowers 338 (FIG. 3) and appropriate aerator piping 340. Mixed liquorreturn weirs 342 (FIG. 3) are also positioned with their intakes at apredetermined top SBR tank water level, corresponding to the “filled”condition in the sequencing batch reaction tank 330. These waste liquidweirs and conduits for returning mixed liquor from the aeration tank tothe anoxic mix tank are important in providing interaction between theaerobic wastewater in the SBR tank 330 and the surge anoxic mix tank324, as previously described.

The SBR treatment tank comprises means for mixing the wastewater liquidsin the tank, and for aerating the liquids in the tank. In theillustrated embodiment, the mixing and aerating functions are carriedout by jet aeration apparatus 120, which comprises a wastewater pump 122for introducing a pressurized stream of wastewater drawn from the anoxicmix tank, into a wastewater manifold of the jet aeration mixer.Pressurized air may be introduced into a pressurized air manifold of thejet aeration mixer by means of a blower in accordance with conventionalpractice. The pressurized liquid and the air may be combined anddischarged into the tank 330 through a plurality of nozzles, which inthe illustrated embodiment are regularly disposed at uniform intervalsalong the pressurized fluid and pressurized air manifold. Thepressurized liquid or pressurized liquid and air stream which isdischarged from the nozzles, is directed toward the opposite wall of thetank, and may be directed downwardly at a slight angle in order to sweepacross the bottom of the tank 102 to thoroughly mix the tank.

The SBR tank 330 accordingly includes solids excluding decanters 342such as model SED18 decanter of Fluidyne Corporation having capacity for2500 gpm each for withdrawing clarified, treated effluent from the SBRtank to an effluent discharge channel 344, where it may be filtered,disinfected, and returned to the environment as a highly purified waterstream.

The hydraulic retention time of the surge anoxic mix tank for typicaldomestic sewage treatment may desirably be in the range of from about 2hours to about 8 hours, while the aerobic sequencing batch aerationtank(s) may desirably have a hydraulic retention time of from about 4 toabout 16 hours, with a total hydraulic retention time (based on theaverage daily influent flow rate) in the range of from about 6 to about24 hours for the total surge anoxic mix and aerobic treatment tankvolume. Preferably, the HRT will be less than 20 hours, and morepreferably, less than 18 hours, such that the total tankage volume maybe less than the total daily treatment volume to be handled by theplant. For example, for a wastewater treatment system designed to treat2,400,000 gallons of wastewater per day, having an HRT of 16 hours, thetreatment volume of the surge anoxic mix and aerobic sequencing batchreactor tanks may have a volume of approximately 1,600,000 gallons, sothat the hydraulic retention time is approximately 16 hours. Because thesolids are retained within the treatment basin for an extended treatmenttime, the solids retention time (SRT) is substantially greater than thehydraulic retention time (HRT).

In accordance with conventional practice in the operation of sequentialbatch reactors, the wastewater solids sludge may be periodicallyrecovered by removal from the bottom of the anoxic mix and/or aerobictreatment basin by a suitable piping system (not shown). In addition,however, the solids, including culture microorganisms may be transferredfrom the aerobic and/or anoxic treatment tanks to the anaerobic trashtrap 316 for total solids reduction, and enhanced phosphate removal aspreviously discussed. Such recovery or transfer from the anoxic mix tankmay be carried out in small amounts with each treatment cycle, or largeramounts at more extended intervals. When phosphate removal is enhancedby transferring wastewater solids to the anaerobic zone, which are atleast in part subsequently introduced into the anoxic zone, it ispreferred to remove the high phosphate sludge from the anoxic or aerobiczone, rather than the anaerobic zone.

At the discharge end of the sequencing batch reactor 330 there isprovided a submerged discharge manifold solids excluding decant system300 such as described in U.S. Pat. No. 4,596,658, which is utilized toperiodically remove a predetermined amount of clarified water from thetreatment basin 330.

In operation, as the surge anoxic mix tank reaches the control waterlevel (CWL), wastewater from anoxic mix tank is pumped to fill SBR1 andcontinue to overfill (interact), while cycling feed pump based on timeand/or DO and/or NO₃ levels in the SBR and surge anoxic mix tanks, untilthe surge anoxic mix tank control water level CWL is reached again.

The feed to SBR1 is then stopped, and the settle timer for SBR1 isstarted while the SBR1 is maintained in quiescent condition forclarification of the upper decant zone. The feed from the surge anoxicmix tank is started to SBR2 and continue as described for SBR1 above.

When the settle timer for SBR1 times out, the decant cycle for SBR1 isinitiated until SBR1 BWL is reached. When the CWL is reached again inthe surge anoxic mix tank, the cycle is repeated.

Illustrated in FIGS. 6 and 7 is an additional embodiment 600 of arelatively compact surge anoxic mix, sequencing batch reactor wastewatertreatment system for a relatively small wastewater flow, such as for aresort area of population 200 to 500, or a small office-industrialcomplex. The design specifications and calculations for the treatmentsystem 600 are set forth in the following Table 3:

TABLE 3 INFLUENT CONDITIONS Flow (mgd) 0.019 Flow (gpm) 32 BOD (mg/l)286 BOD (lb/d) 45 TSS (mg/l) 1300 TSS (lb/d) 206 TKN (mg/l) 163 TKN(lb/d) 26 OXYGEN REQUIREMENTS Lbs. TKN required for synthesis 2.27 Lbs.NO3-N produced 24 Lbs. 02 recovered/lb NO3-N reduced 2.6 Lbs. Oxygen/lb.of BOD 1.4 Lbs. Oxygen/lb. TKN 4.6 Actual Oxygen Demand (lb 02/d) Total111 Alpha 0.9 Beta 0.95 Theta 1.024 Operating Dissolved Oxygen (mg/l) 1Clean Water Oxygen Sat. at Op. Temp (Mg/l) 10.07 Clean Water oxygen sat.at Std. Temp (mg/l) 9.09 Clean Water 02 sat, std temp, mid depth (mg/l)10.49 Std. condition ambient pressure (psia) 14.7 Oper. conditionambient pressure (psia) 11.65 Wastewater temperature (c) 15 SOR/AORratio 1.69 Standard Oxygen Demand (lb 02/d) total 187 Standard OxygenDemand (lb/02/hr) 15.44 INFLUENT CONDITIONS Specific oxygenation rate(mg/l-hr) 53 Lbs. of oxygen/lb. of air 0.23 Clean Water Efficiency (%)15 Lbs. of Air/Cubic Ft. of air 0.075 Aeration hours per day 12.12 Airflow rate (SCFM/tank) 99 NITRIFICATION/DENITRIFI- CATION Requiredalkalinity for nitrification (mg/l) 1062 Alkalinity recovered,denitrification (mg/l) 446 Net alkalinity required (mg/l) 616 Mixedliquor temperature, C. 15 ML dissolved oxygen (mg/l) 1 Max. nitrifiergrowth rate, day-1 0.204 Minimum SRT required for nitrification, days4.89 Actual or Design SRT, days 83.63 Kn, half velocity constant (mg/l)0.40 Design growth rate for heterotrophs/nitrifiers 0.0120 Projectedeffluent soluble NH3-N, mg/l 0.03 Specific utilization rate, lbs BOD5/lbmlvss 0.11 lbs. mlvss required for BOD & NH3 removal 402 mlvss (mg/l)3250 Tank volume req. for BOD & NH3 removal (MG) 0.015 Aerobic hrs/dayrequired, hr. 10.12 Denitrification rate (g/g/day) 0.043 lbs mlvssrequired for denitrification 551 Tank volume required for NO3 removal(MG) 0.020 Anoxic hrs/d required/hr. 13.86 Anoxic mix hrs/d 9.69INFLUENT CONDITIONS Total tank volume required (MG) 0.0352 SBR TankConfiguration No. of tanks 1 Length overall (ft) 32 Length Surge AnoxicMix tank (ft) 10.70 Length SBR tank (ft) 21.30 Width (ft) 14 Bottomwater level (ft) 8.4 Top water level (ft) 10.5 Top of Wall (ft) 12 No.decanters/tank 1 Total tankage volume a TWL (MG) 0.0352 HRT (hrs) 44.45CYCLE TIMES/CAPACITY CALCULATIONS Total Decant Volume (cubic feet) 626Total Decant Volume (gallons) 4,684 Decant volume per tank (gallons)4,684 Number of cycles per day/tank 4.06 Total time per cycle (minutes)355 Fill rate (gpm) 742 Fill time (minutes) 6.31 Interact period (min)287 Settle period (minutes) 50 Average decant rate (gpm/ft decanter) 100Decanter length (feet) 4 Decanting time (minutes) 12 Decanting rate(gpm) 400 Peak decanting rate (gpm at start of decant) 440 Idle periodtime (minutes) 0 INFLUENT CONDITIONS Zero idle & react time, flow rate(MGD) 0.109 Peak/average flow 5.762 Maximum aeration period available(hours/day) 19.83 EQUIPMENT SELECTION Air flow per nozzle (scfm) 100Number of nozzles required (per tank) 0.99 Number of nozzles provided(per tank) 2 Actual airflow per nozzle required (scfm) 49.74

In the treatment system 600, the influent wastewater to be treated iscontinuously introduced into trash trap tank 602, which is generally inan anaerobic condition with facilitative anaerobes predominating in thetank 602. The influent flows from the trap tank 602 to the anoxic mixtank 604 having a nominal (filled) volume of 20,000 gallons, whichcontains two motive pumps 606 of 10 horsepower each, with a fluid intakenear the bottom of the tank 604. Each motive pump output is directedthrough conduit 608 to a jet aspiration aerator 610 (FluidyneCorporation Model FJASQ4 which is directed into the sequencing batchreaction aeration tank 612, which has a nominal (filled) volume of35,000 gallons. The aeration tank also has an overflow weir assembly 616which drains mixed liquor at a top water level (here 10.5 ft) back intothe anoxic mix tank as most clearly shown in FIG. 7. The SBR aerationzone also includes a decanting system 618 such as the Fluidyne SED6Decanter, which is adapted to remove a predetermined portion of an upperclarified layer and removed from the treatment zone by withdrawingclarified water through a decanting orifice which is orientedhorizontally along its length in the treatment zone. The decanter 618functions, when operated to drain the aeration tank from its top waterlevel of 10.5 feet, to its bottom water level of about 8.4 feet.

The decanting step may be controlled by opening and closing a singlevalve in the decanter 618 and permitting hydraulic pressure to force theclarified liquid through the horizontally extended decantation orifice.The decantation step may be initiated by removing the air from the fluidtrap zone to establish a continuous liquid column in the hydraulic trapzone. The treated decanter 618 outflow is directed to an effluentdischarge chamber 620, from which it may be further filtered,disinfected, and discharged to the environment.

As also indicated, various apparatus aspects of the present inventioncomprise a sequential batch reaction tank, and tank inlet means forintroducing wastewater to be treated into the tank, together withhorizontally extended decantation means for withdrawing liquid from thetank positioned within the tank at a predetermined height substantiallycorresponding to a minimum predetermined water level decantation height.

Aeration and other biological treatment tanks, including continuoussystems, sequencing batch reactors, and surge anoxic mix treatmentsystems such as those of FIGS. 1-7, may have problems of foam and scumaccumulation on the surface of the tank contents. Foam is typicallypresent at start up of the biological process. During aeration,surfactants contained in the influent wastewater will produce foam untilsufficient bacteria growth and biological activity biodegrade thesurfactants, to a degree sufficient to suppress foam generation. As thewastewater treatment bacteria grow and age, scum may accumulate on thetank water surface. This scum typically consists of bacteria and otherbiological growth such as nacardia and other antinamycetes or fungi.Other floatables such as plastics and paper may be incorporated in thescum. The scum is unsightly and can be a source of odor. Scum can alsointerfere with proper operation of mechanical equipment such asdecanters, clarifiers, and sludge holding and processing tanks andequipment.

Though there are a variety of commercially available or custom made scumskimmers, they have proven less than fully effective in removing scum.This is especially true in aeration tanks where scum skimmers typicallyonly remove the scum from a limited surface area at any one time,involve limited wastewater removal with the scum, and are affected bywind and wave action.

Certain aspects of preferred systems in accordance with the presentinvention are directed to more complete and effective scum removal. FIG.8 schematically illustrates an appropriate weir assembly 802 disposed tocontrol flow between an aerobic treatment zone 806 and an anoxictreatment zone 804 of a surge anoxic mix treatment system such as shownin FIGS. 1-7. Schematic views of both an interact phase of a treatmentcycle (in which fluid is introduced from the anoxic mix zone 804 intoaerobic treatment zone 806 while fluid overflows from the zone 806 backinto the zone 804), and a decant phase of a treatment cycle (in whichfluid from the anoxic mix zone 804 is introduced into the quiescentaerobic treatment zone 806 without substantial mixing), are shown inFIG. 8. As indicated in FIG. 8 together with FIGS. 9, 10A and 10B, byoverflowing tank contents through a multi-purpose flow and scumcontroller 802, foam and scum may be effectively removed from the tankand concentrated in another tank. By proper weir arrangement as depictedin FIGS. 9-10, rising water can be caused to overflow the weir at asubstantial rate carrying scum and other floatables into a separateconcentrating and holding tank, or to an “upstream” zone of thetreatment process. If directed to an upstream treatment zone, the scumwill be subjected to further biological treatment. If directed to aseparate holding tank, the scum breaks down under long termbiodegradation, eliminating further treatment of the scum. Nonbiologicalor inert material incorporated with or skimmed with the scum can then beeasily removed in any appropriate manner, such as by separate drain orsuction systems. The hydraulic head available from the overflowing tankcontents may be used to mix the concentrating and holding tank. Asimilar set of overflow weirs may also be used to contain and diffusehigh flow passing through the tank combination. Flow exits the holdingtank beneath the scum layer in reverse flow fashion and enters theskimmed tank through a diffusion shroud so as not to disturb the tankduring settle and decant operations.

A weir assembly 802, which serves as a fluid transfer mechanism betweenan aerobic treatment zone 902 and an anoxic treatment zone 904 separatedby wall 906, is shown in FIG. 9. The weir assembly 802 comprises anouter flow baffle consisting of a first baffle 810 which is parallel tothe tank wall 906, two side external baffles 812, 814 which join theparallel baffle 810 to the wall 906, an internal flow baffle consistingof baffles 820, 822 and 824 which form an internal U-shaped enclosure,and a conduit 826 extending from the bottom of the U-shaped enclosure,downward, and through the tank wall 906 from the zone 902, to the zone904. During the settle and decantation phase of the sequential treatmentcycle, it is important that the introduction of fluid from the anoxicmix tank or zone, or any other source, not disturb the settled effluent,and not “short circuit” to the effluent outlet stream. In this regard,it is generally not desirable to introduce fluid from an anoxic mix zoneinto the aeration zone during the settling and decantation portion ofthe treatment cycle. However, during high influent flow conditions, thetreatment capacity of the system may be exceeded, and it may beaccordingly necessary to pass through some of the partially treatedwastewater in the anoxic zone into the aeration zone, during thesettling and decantation phase. As shown in FIG. 10A the weir apparatus802 effectively accomplishes flow baffling and diffusion undercircumstances in which flow may be introduced from the anoxic mix tankto the aeration zone, where the settling and decantation are occurring.As shown in FIG. 10A, when the hydraulic level in the anoxic mix zoneexceeds the top of baffle plates 820, 810, 824, the flow introduced bythis hydraulic head passes into the shrouded zone formed by baffleplates 812, 810, 814 surrounding the internal baffle and conduit 826.The waste fluid thus is introduced into the aeration zone with a slightdownward momentum, into the lower, settlement zone containing thesettled solids and bacteria, and away from the upper, clarified zonefrom which treated, clarified effluent is removed. In this manner, shortcircuiting of influent and or anoxic mix tank contents is prevented.Baffling, stilling and flow direction are provided, and the influent isdiffused into settled sludge to enhance treatment at high flows. Again,with reference to surge anoxic mix treatment systems such as thoseillustrated in FIGS. 1-7, during the interact phase of the treatmentcycle, the weir apparatus 802 performs the scum skimming and mixedliquor recycle functions as previously described. In this regard, duringthe interact phase, waste liquid from the anoxic zone is introduced intothe aeration zone until the hydraulic level in the aeration zone reachesthe top level of the weir 802. This is accompanied by concomitantlowering of the hydraulic level in the anoxic mix zone (after alsoaccounting for influent wastewater). As shown in FIG. 10B, when thehydraulic level in the aeration tank reaches the top of the weir formedby the baffles 820, 810, 824, the wastewater at the surface of theaerobic zone adjacent the weir, including any scum and/or foam, istransferred down the conduit 826, through the wall 906, and into theadjacent anoxic mix zone. As wastewater continues to be pumped into theaeration zone (through a separate pump as previously described), thesurface liquid, and any surrounding scum, surface debris and/or foamcontinue to be transferred to the upstream anoxic mix zone, forcontinued treatment.

As indicated, some embodiments of the present disclosure are alsodirected to integrated wastewater treatment systems which have reducedlevels of sludge production, and/or independent or “stand-alone” systemsin which wastewater biological treatment sludge is treated to reduce itsvolume and bulk, particularly including its organic content. Thus, thesludge reduction capabilities described herein can be integrated in thedesign of the previously described surge anoxic mix, sequential batchreactor, wastewater treatment systems. In such systems, the influentwastewater to be treated is introduced into an anaerobic treatment zone,where at least a portion of the total suspended solids of the influentwastewater is settled to an anaerobic settled solids zone in the lowerportion of the anaerobic treatment zone. Waste liquor from the anoxicand/or aerated treatment zones containing microbial sludge produced bythe anoxic and aerobic treatment processes is recycled to the anaerobictreatment zone, wherein at least about 50 percent by weight of themicrobial sludge and other solids content (TSS) of the waste liquorrecycled to the anaerobic zone is settled to the anaerobic settledsolids zone in the lower portion of the anaerobic treatment zonetogether with settled influent wastewater solids. Under the anaerobicdigestion conditions, the mixture of the settled mixture of the rawinfluent solids and the recycled microbial sludge solids isanaerobically digested. Typically at least about 50 percent by weight ofthe influent organic solids and the recycled microbial solids whichsettle in the settled solids zone are anaerobically biologicallydigested to produce anaerobically digested solid, soluble and gascomponents in the anaerobic treatment zone. Wastewater from theanaerobic treatment zone, which includes both influent wastewater andrecycled waste liquor wastewater, is conducted from the anaerobic zoneto the anoxic zone, and carrying with it the soluble anaerobic digestioncomponents for anoxic biotreatment in the anoxic zone, and aerobicbiotreatment in the aerobic treatment zone. Similarly, independentsystems for treatment of sludge from independent biotreatment or sludgetreatment systems, such as aerobic or anaerobic digesters or activatedsludge systems, can be provided in order to reduce the amount of sludge,from such digesters or other sludge source, which must be disposed of bylandfill or longer-term treatment processes. As examples of suchintegrated sludge-reduction systems and independent sludge reductionsystems, illustrated in FIGS. 11-14 are systems which integrateanaerobic, anoxic, and aerobic processes for raw sewage treatment and/orfor sludge management, reduction, and consumption or destruction. Suchsystems of should be capable of reducing the amount of organic sludgefor ultimate disposal by 80% or more, by weight, as compared to thesludge produced in the absence of such features. In this regard, theamount of organic sludge conventionally produced prior to aerobic oranaerobic digestion will typically be about 0.4 to about 0.8 grams pergram of BOD5 in the influent wastewater to be treated. The system of thepresent disclosure utilizing anaerobic/anoxic sludge recycle willpreferably reduce the amount of organic sludge produced by the system toless than about 0.2 grams of organic sludge per gram of influent BOD5,and preferably less than about 0.1 grams of organic sludge per gram ofBOD5.

In systems such as illustrated in FIGS. 11-14, raw or pretreated sewageis introduced into a first anaerobic zone, which can correspond to trapzone 316 of the system of FIG. 3, or the trap zone 602 of the system ofFIG. 6. The anaerobic zone is unmixed or lightly mixed such thatinorganic and fast-settling organic solids settle to the bottom of thezone and are concentrated at the bottom of the zone. Settled andunsettled organic solids undergo anaerobic digestion, consuming organiccomponents and producing products of anaerobic digestion such as carbondioxide, methane, ammonia, hydrogen sulfide, and organic intermediatesor breakdown products. Many of these anaerobic digestion products aresoluble and pass with the liquid phase into the next anoxic/aerobiczone, and subsequent microbial treatment under respective anoxic andaerobic conditions as previously described. Some of the byproducts arevolatilized and pass into the gas phase. The off-gas can be treated forodor control as required (see FIG. 14 showing a covered anaerobictreatment tank), in accordance with conventional practice, such as bypassage or filtration through a basic absorbent. The surface loadingrate of the anaerobic zone is preferably from about 100 to 1000gallons/ft²/day (4 to 40 M³/M²/day). The organic loading rate ispreferably from about 60 to 300 pounds/1000 ft³/day (1 to 5 kg/M³/day).

Wastewater flow passing through the anaerobic zone carries solids,organics, and nitrogen-containing compounds on to subsequent treatmentin anoxic and aerobic zones as previously disclosed, and as shown inFIGS. 11B and 12. The waste sludge produced in anoxic and/or aerobiczones is recycled to the anaerobic zone for further digestion. Organicsolids in the anaerobic zone, are continuously degraded and consumed aspreviously described, washed of organics or elutriated by the flowthrough the anaerobic zone, leaving the heavier inorganic solids in theanaerobic zone. The heavy inorganic solids concentrate at the bottom ofthe anaerobic zone with the biologically more inert organics, where theywill continue to slowly be degraded by anaerobic processes, and canperiodically be removed. Unlike pure aerobic digestion, however, highersolids concentration is possible by use of systems in accordance withthe present disclosure. At 80% VSS reduction and 5% residual solidsconcentration, only 2,000 gallons of residual solids per day are leftfor disposal from the previously discussed example of a hypothetical 1MGD plant. This amounts to only about 0.2% of influent flow. Illustratedin FIG. 11A is a typical solids balance for “conventional” wastewatertreatment. As shown in FIG. 11A, influent wastewater 1102 typicallycomprising 0.02 percent by weight total suspended solids (TSS), of whichabout 85 percent of the total suspended solids may typically be volatilesuspended solids (VSS). The remaining 15 percent of the total suspendedsolids is inert, non-biodegradable inorganics or other fixed suspendedsolids (FSS). The influent wastewater 1102 is processed by thebiotreatment system 1104 to produce treated effluent 1106, in which thetotal suspended solids is reduced to about 0.002 percent by weight,which is substantially 100 percent volatile suspended solids (VSS). Inthis regard, substantially all of the inert, inorganic or otherwisefixed suspended solids (FSS) are removed from the treated effluentstream 1106 by conventional biotreatment. The biotreatment system 1104also produces a waste sludge stream 1108 which may typically compriseabout 0.2 to about 1 percent total suspended solids, of which about 70weight percent is volatile suspended solids, and the remaining 30percent is the inorganic, inert, fixed suspended solids of the influentwastewater stream 1102. Typically, the biotreatment waste stream issubjected to aerobic digestion in an aerobic digestor 1110 to furtherreduce pathogens and volatile suspended solids content. The sludgestream 1112 from the aerobic digestor will typically have a relativelyhigher total suspended solids content of about 2 weight percent, ofwhich about 60 percent is still undigested volatile suspended solids,the remaining 40 percent being the fixed suspended solids content of theoriginal influent stream 1102. The sludge stream 1112 from the aerobicdigestor 1110 will conventionally be dewatered by centrifuge or othersuitable dewatering system 1114 to produce a sludge cake 1116 containingabout 15-20 percent total solids, of which about 60 percent by weightremains as volatile suspended solids content. This sludge cake 1116requires disposal and/or subsequent treatment, as previously described.The water stream 1118 separated by the dewatering system may bereintroduced into the biotreatment system 1104, as is the clarifiedeffluent from the aerobic digester 1110.

Sludge reduction systems of the present disclosure can provideconsiderable reduction in the solids which are produced by a wastewatertreatment system, as illustrated by FIG. 11B, which is a typical solidsbalance for sludge reduction systems utilizing recyclic anaerobictreatment of the wastewater sludge produced by the biotreatment of theinfluent wastewater. As illustrated in FIG. 11B, the influent wastewaterstream 1103 will similarly typically comprise about 0.02 percent byweight total suspended solids, of which about 85 percent of such totalsuspended solids may typically be volatile suspended solids components.Again, the remaining 15 percent of the total suspended solids of theinfluent wastewater stream 1103 is inert, non-biodegradable inorganicsor other fixed suspended solids. In the recyclic anaerobic system ofFIG. 11B, the influent wastewater stream 1103 is introduced into ananaerobic treatment zone 1105. In the anaerobic zone 1105, the influentwastewater (and other sludge components introduced into the anaerobiczone, as will be more fully described) is subjected to anaerobicdegradation, which produces low molecular weight soluble and volatilecomponents such as carbon dioxide, methane, ammonia and hydrogensulfide, together with soluble organic compounds, as previouslydiscussed. The wastewater stream 1107 exiting the anaerobic treatmentzone 1105 may typically comprise about 0.1 percent total suspendedsolids, of which about 95 percent is volatile suspended solids, and only5 weight percent is fixed suspended solids. The anaerobic effluentstream 1107 is introduced into the anoxic treatment zone 1109, and fromtheir into an aerobic treatment zone 1111 of a surge anoxic mix,sequencing batch reactor system such as previously described. Because ofthe retention of wastewater treatment organisms in the sequential batchreactors system, the wastewater streams 1113 and 1115, which aresequentially recirculated between the anoxic treatment zone 1109 and theaerobic treatment zone 1111 similarly comprise from about 0.2 to about0.5 percent by weight of total suspended solids, approximately 95percent of which is volatile suspended solids. The treated wastewaterstream 1117 discharged from the aerobic treatment zone 1111 hasapproximately only about 0.001 percent total suspended solids,substantially all of which is volatile suspended solids. As indicated,the sludge reduction systems of the present disclosure utilize recyclicanaerobic treatment of the sludge produced by the wastewater treatmentprocess. In this regard, a wastewater stream 1119 is conducted from thesurge anoxic mix treatment system (from either or both of the anoxic mixtank 1109 or the aerobic treatment tank 1111). It should be noted thatthe wastewater liquor in the aerobic treatment zone will have a somewhathigher solids content after the decantation step, which produces theclarified effluent by settling the suspended solids. If the stream 1119is selected from the sediment and liquid remaining at the conclusion ofthe decantation step from the aerobic treatment zone 1111, the totalsuspended solids content may be somewhat higher, and the liquid volumenecessary to transport this solids content may be somewhat lower.However, this may require a separate pump, and also introduces moreaerobic liquor into the anaerobic zone. Preferred embodiments of thesystem accordingly may involve a continuous recycling of the wastewaterliquor from the anoxic treatment zone 1109 to the anaerobic zone 1105.In any event, the total suspended solids content of the wastewaterliquor 1119 returned to the anaerobic treatment zone 1105 is typicallyat least about 0.3 times the amount of the total suspended solids orBOD5 content of the influent water 1103 to the treatment system.Accordingly, the flow rate of the liquor stream 1119 continuously orintermittently recycled to the anaerobic zone 1105 from the anoxicand/or anaerobic zones 1109/1111 will typically be in the range of fromabout 50/1 to about 1/300 of the influent 1103 flow rate of waste waterto be treated. (100-200% more typical) Upon entering the relativelyquiescent anaerobic treatment zone 1105, the microbial and other solidscontent of the recycled liquor 1119 tends to settle to the bottom of theanaerobic treatment tank 1105, where they undergo anaerobic digestionand partial conversion to gases and more soluble components, aspreviously discussed. Periodically, or continuously, a stream 1121 ofrelatively high solids content, of from about three to about fivepercent by weight total suspended solids, of which about half isvolatile suspended solids and the remaining half is fixed suspendedsolids, may be discharged from the anaerobic zone 1105 for furthertreatment and disposal. It should be noted that the reduction andvolatile suspended solids represents a significant economic savings insubsequent treatment requirements. In integrated systems where thesludge reduction step is incorporated in the liquid processing steps,the anaerobic zone volume is typically 20 to 40% of the totalanaerobic+anoxic+aerobic volume and is typically set or controlled bythe settling requirements and organic concentration of the influentsteam. For domestic sewage, the preferred surface loading rate is fromabout 300 gal/ft³ of anaerobic zone/day to about 600 gal/ft² ofanaerobic zone/day. For industrial wastes having high organicconcentrations or separate sludge reduction systems where influent flowis handled in a separate system and organic loading is concentrated inthe sludge stream to be treated, the anaerobic zone is typically alarger percentage of total volume. The anaerobic zone volume in thiscase is typically set to provide an organic loading rate of from about 2kg BOD5/M³ to about 6 BOD5 kg/M³.

Residual solids may be reduced even further by additional separationprocesses. Illustrated in FIG. 12 is a surge anoxic mix wastewatertreatment system with integrated sludge reduction, provided by wasteliquor recycle through anaerobic, anoxic, and aerobic treatment zones.In this regard, the wastewater influent 1202 to be treated is introducedinto a relatively quiescent, anaerobic treatment zone 1204, where theinfluent suspended solids can settle and be anaerobically digested. Theeffluent 1206 from the anaerobic treatment zone, which containssolubilized organic compounds produced by the anaerobic digestion, isintroduced into the anoxic treatment zone 1208, where it undergoesanoxic treatment to consume soluble organic materials and releasenitrogen, and is pumped to the aerobic treatment zone 1210 by pump 1212as an anoxic stream 1214. As previously described, the waste liquor inthe aerobic treatment zone 1210 is subjected to periodic aerobictreatment, recycle to the anoxic zone, settling and decantation of aclarified, treated effluent stream 1216. The anoxic 1208 and aerobic1210 zones are integrated with the anaerobic zone 1204 by recycling of awaste liquor stream 1222 to the anaerobic zone 1204 from the anoxicand/or aerobic zones, as previously discussed, particularly inconnection with FIG. 11B. In order to concentrate the VSS forbiodegradation, a screen 1218 separates relatively larger, solid, inertmaterials such as pieces of plastic which will not be biodegradable, andremoves them as a relatively inert output component 1220. The screenedwaste liquor stream 1224 is returned to the anaerobic zone as shown inFIG. 12A. In the anaerobic zone 1204, a portion 1226 of the settledsolids at the bottom of the anaerobic zone is periodically pumpedthrough a grit separator cyclone 1228, which may be of conventionaldesign such as the model #FHG1 Hydrogrit (TM) grit separator of FLUIDYNECorporation of Cedar Falls Iowa, and is returned to the anaerobic zonein a relatively quiescent manner. The grit stream 1230 which is removedby this treatment will be relatively high in inorganic components suchas sand and clay, which are more readily dewatered and disposed of inthe landfill than wastewater filter cake. Such separator screen and gritcyclone treatment may be applied at various other streams in theintegrated system. The organic sludge VSS components are retained in thesystem for extended sludge reduction, but excess sludge may beperiodically removed in accordance with conventional practice whennecessary. A dual surge anoxic mix treatment system like that of FIGS. 2and 3, with sludge reduction processing design, is similarly shown inFIG. 12B. The wastewater influent stream 1252 is introduced to anunstirred anaerobic treatment tank 1254, which discharges to a singleanoxic tank 1256 interacting with two aerobic tanks 1258, 1260. Wasteliquor from the anoxic tank 1256 is recycled to the anaerobic tankthrough screens 1262, 1264 to remove larger, non-biodegradablematerials, and the anaerobic solids may be cycled through an inorganicgrit separator 1266 to remove FSS 1268, as previously discussed.

For small treatment plants, the anaerobic zone can also be used forpretreatment by trapping trash and grit, and washing them for ultimatedisposal. Larger plants may generally utilize separate pretreatment. Inthis regard, such systems may accordingly have an additionalpretreatment tank to which sludge is recycled, and which discharges tothe anoxic mix tank. The sludge in the tank may be recycled through ascreen and/or cyclone filter to remove inert solids, as described andshown in FIGS. 12-14.

Treatment systems in accordance with the present invention can also beused for treatment and reduction of sludge from existing or separatebiotreatment plants. In this application, the preferred embodimentincorporates continuous or periodic sludge screening and inorganicsremoval, to remove and concentrate inorganics and substantiallycompletely oxidize the remaining organic sludge. As shown in FIG. 12C(Sludge Reduction System, and SRS with inorganic removal), waste sludgeor mixed liquor 1282 from the biotreatment plant or sludge holding tanks1284 may be passed through a fine screen (0.010 to 0.100 slot opening)1286 and all or part of the screened sludge is introduced into theunstirred anaerobic zone 1288. The balance is recycled to thebiotreatment plant and/or aerobic digester 1290. A recycle pump 1292takes inorganic containing sludge from the anaerobic or anoxic zone andpasses it through a cyclone 1294 to continuously remove the inorganics.The fine screen removes bits and pieces of plastics, which may beorganic in nature but resistant to biodegradation and therefor may beregarded as being essentially inert. The fine screen may also removelarger inorganic particles. The hydrocyclone may remove inorganicparticles as fine as 25 microns in major dimension. The rejects from thescreen and hydrocyclone are essentially dewatered with solidsconcentrations of 50% or more. The continuous removal of inorganicsallows additional room for the remaining organic solids in the anaerobictreatment zone 1288, allowing additional processing time andbiodegradation. Given adequate time and conditions the organic sludge isultimately substantially consumed and destroyed. The principal residualproducts then become the screenings and removed inorganics, which aresuitable for collection in dumpsters for ultimate land fill disposal.Since the screenings and inorganic solids have gone throughbiotreatment, they are relatively stable and unobjectionable forlandfill purposes. Similarly illustrated in FIG. 12D is an independentsludge reduction system adapted to use surge anoxic mix sequential batchreaction systems as described herein, to process waste sludge 1281 fromconventional biotreatment or sludge holding tank 1283. As shown, thewaste sludge may be first screened to separate larger nonbiodegradablecomponents such as plastic pieces, for disposal to a portable dumpsteror other receptacle, and introduced directly into the anoxic (oranaerobic) treatment tank 1285 of a sludge reduction system alsocomprising a relatively quiescent anaerobic treatment tank 1287, and anaerobic treatment tank 1289. The waste liquor is recycled through a gritcyclone from the anoxic 1285 and/or aerobic tank 1289 to the anaerobictreatment tank, after clarification of effluent 1292 from the tank(which can be introduced as influent wastewater to a wastewatertreatment system). The inorganic grit component is relatively benign fordisposal purposes, and its removal permits more efficient bioprocessingof the anaerobic treatment tank, where the inorganic grit wouldotherwise accumulate. In the independent sludge reduction system of FIG.12D, the waste sludge 1281 introduced into the system typically has arelatively high solids content (e.g., at least about 0.2% by weightsolids), and the system is particularly adapted to reduce the VSScomponents of the sludge. In this regard, the recycle flow rate of thewaste liquor pumped from the aerobic tank 1289 (or the anoxic tank 1285)will typically be at least about 50 percent of the flow rate of wastesludge 1281 introduced into the anoxic (or anaerobic) tank forprocessing. For municipal wastewater sludge, this recycle flow rate maydesirably be in the range of from about 0.5 to about 5 times theinfluent flow of waste sludge 1281. The anaerobic tank 1287 willtypically constitute from about 20 to about 60 percent of the totalvolume of the anaerobic, anoxic and aerobic tanks 1285, 1287, 1289.Similarly, the anoxic tank 1285 will typically comprise from about 10 toabout 40 percent, and the aerobic tank 1289 will typically comprise fromabout 20 to about 60 percent of the total processing, or tankage, volumeof the anaerobic, anoxic and aerobic tanks together, in order tomaximize the sludge reduction capability of the system.

Assuming substantially all of the inorganic suspended solids are removedand disposed of at 50% solids content, and that there are 10% residualorganic solids at 5% solids concentration, a total of only about 400 gpdof residuals require ultimate disposal from the 1 MGD example above.This is well below 0.1% of influent flow, and represents better than a10 to 1 reduction in residuals for ultimate disposal as compared toconventional, standard wastewater treatment systems.

Illustrated in FIGS. 13 and 14 is an independent sludge reduction system(SRS) 1300 which is used in conjunction with an adjacent wastewatertreatment system (partially shown in FIG. 13) to reduce the total wastesludge output from the wastewater treatment system. As shown in FIG. 13,which is a top view of the SRS system 1300 adjacent a sequential batchreactor tank 1302 of the wastewater treatment system, and adjacentseparate conventional aerobic digestor tank 1304 for the wastewatersystem.

The sludge reduction system 1300 comprises an anaerobic tank 1308, ananoxic tank 1310 and an aeration tank 1312 shown approximately to scale,which are generated as previously described. The aeration tank 1312 isboth mixed and aerated by jet aspirators 1314 and can receive andprocess waste sludge from either or both of the aerobic digestor 1304 byappropriately introducing the sludge in the system. In this regard,waste sludge 1306 from the SBR tank 1302 or the aerobic digestor 1301,may be introduced into the covered anaerobic treatment tank 1308 througha screen 1316 which filters out larger, generally inert particles. Thescreen 1316 may discharge into any of the tanks 1308, 1310, 1312, butpreferably the anaerobic or anoxic tank. The heavy settled solids fromthe bottom of the anaerobic tank 1308, which will typically have highinorganic content, may be pumped through a grit cyclone 1318 for gritremoval and return to the anaerobic treatment tank or the aerobicdigestor.

While the present invention has been described with respect toparticular embodiments of apparatus and methods, it will be appreciatedthat various modifications and adaptations may be made based on thepresent disclosure and are intended to be within the scope of theaccompanying claims.

What is claimed is:
 1. An integrated sludge reduction method forbiooxidation of wastewater comprising the steps of: providing ananaerobic wastewater treatment zone, an anoxic waste water treatmentzone, and one or more aerobic wastewater treatment zones, wherein thevolume of the anaerobic treatment zone comprises from about 20 to about40 percent, the volume of the anoxic treatment zone comprises from about10 to about 40 percent, and the volume of the one or more aerobictreatment zones comprises from about 20 to about 60 percent of the totalprocessing volume of the anaerobic, anoxic and aerobic tanks,introducing wastewater having BOD5 and suspended solids to be treatedinto the anaerobic treatment zone and settling at least a portion of thesuspended solids to an anaerobic settled solids zone in the lowerportion of the anaerobic treatment zone, circulating fluid from theanaerobic zone to the anoxic treatment zone, from the anoxic treatmentzone to the aerobic treatment or zones, and from the anoxic treatmentzone and/or the aerobic treatment zone(s) back to the anaerobictreatment zone, wherein at least about 50 percent by weight of themicrobial sludge and other solids content (TSS) of the waste liquorrecycled to the anaerobic zone is settled to the anaerobic settledsolids zone in the lower portion of the anaerobic treatment zonetogether with settled influent wastewater solids, anaerobicallydigesting at least 50% by weight of the settled mixture of raw influentsolids and the recycled microbial sludge solids to produce anaerobicallydigested solid, soluble and gas components in the anaerobic treatmentzone which are conducted to the anoxic and aerobic treatment zones formicrobial oxidation, and discharging treated water from the aerobictreatment zone.
 2. A method for sludge reduction in accordance withclaim 1, wherein said fluid circulation is carried out at a recycle flowrate of the waste liquor pumped from the aerobic tank or the anoxic tankto the anaerobic tank of from about 0.5 to about 5 times the influentflow of waste sludge or mixed liquor, and further comprising filteringwaste liquor from the bottom of the anaerobic treatment tank to remove arelatively heavy inorganic solids fraction having a solids concentrationof at least 50%.
 3. A sludge reduction method for biooxidation ofwastewater sludge comprising: providing biotreatment plant waste sludgeor mixed liquor; providing an anaerobic waste water treatment zone, ananoxic waste water treatment zone, and one or more aerobic waste watertreatment zones, wherein the volume of the anaerobic treatment zonecomprises from about 20 to about 60 percent, the volume of the anoxictreatment zone comprises from about 10 to about 40 percent, and thevolume of the one or more aerobic treatment zones comprises from about20 to about 60 percent of the total processing volume of the anaerobic,anoxic and aerobic tanks, introducing said waste sludge or mixed liquorinto the anaerobic treatment zone at an organic loading rate of fromabout 2 kg BOD5/M³ to about 6 BOD5 kg/M³ of the anaerobic treatment zonevolume, circulating fluid from the anaerobic treatment zone to theanoxic treatment zone, from the anoxic treatment zone to the aerobictreatment zone or zones, and from the anoxic treatment zone and/or theaerobic treatment zone(s) back to the anaerobic treatment zone; anddischarging water to a primary wastewater treatment system.